1. Field of the Invention
The present invention relates to an intake air quantity measuring apparatus, and more particularly to an apparatus for measuring a quantity of the intake air which flows into an intake air duct of an internal combustion engine.
2. Description of the Related Art
For measuring a quantity of the intake air supplied to an internal combustion engine, various air quantity measuring apparatuses have been employed. In the prior apparatuses, an intake air temperature sensing element and a flow speed sensing element are disposed in an intake air duct with the planes of the elements parallel to the flow of the intake air, as disclosed in Japanese Patent Laid-Open Publication No. 60-230019 for example. Both elements comprise heat sensing resistors which form a bridge circuit with fixed resistors. In parallel with the flow speed sensing element, there is provided a heating resistor which heats up the heat sensing resistor of the flow speed sensing element to a higher temperature than that of the intake air temperature sensing element by a predetermined temperature difference. Then, the heating current supplied to the heating resistor is regulated in response to the temperature of the heat sensing resistor of the flow speed sensing element, which temperature is changed in response to the quantity of heat transferred to the intake air, so as to maintain the predetermined temperature difference. Consequently, the flow speed of the intake air is measured through the heating current, and the quantity of the intake air is determined by the flow speed.
The above-described intake air quantity measuring apparatus is of a so-called indirect heating type. Further, in Japanese Utility Model Laid-Open Publication No. 60-183825, there is disclosed an apparatus having an element forming all the resistors including the heating resistor with a thin film resistor. An apparatus of a so-called self-heating type is also known, as disclosed in Japanese Patent Laid-Open Publication No. 57-201858 in which the flow speed sensing element is provided with a cantilever base plate having a thin film resistor deposited thereon. Further, in Japanese Patent Laid-Open Publication No. 60-236029, there is disclosed another self-heating air quantity measuring apparatus which is provided with a base plate having the film resistor deposited thereon both ends of which are fixed to the intake duct. In this publication, it is proposed that the ohmic value of the film resistor at the upstream in the intake air duct is set larger than that at the downstream so that the quantity of heat generated at the upstream is more than the quantity of heat at the downstream, whereby a temperature distribution of the film resistor is made uniform.
With respect to the speed sensing element in the air quantity measuring apparatus, the temperature of the element must be changed in response to the flow speed of the intake air. Therefore, if the speed sensing element itself or other factors relating thereto affects the above-described temperature change, it is difficult to measure the quantity of the intake air accurately. Therefore, any of the base plates of the flow speed sensing elements in the above-described publications is made into a thin flat plate which is disposed in parallel with the air flow of the intake air.
In the air quantity measuring apparatus as described above, the quantity of heat generated by the flow speed sensing element is transferred to the air surrounding it or to a portion of the intake air duct to which the base plate is fixed. In the case where the flow speed of the intake air is high, almost all the quantity of heat is transferred to the surrounding air. On the contrary, in the case where the flow speed of the intake air is low, the quantity of heat transferred to the surrounding air is reduced, whereas the quantity of heat transferred to the portion of the base plate fixed to the intake air duct is increased relatively. Since this quantity of heat transferred to the portion of the base plate fixed to the intake air duct is changed depending upon the temperature of the intake air or that of the intake air duct, the current fed to the above-described bridge circuit is changed, so that the accuracy of sensing the flow speed of the intake air is deteriorated. In Japanese Patent Laid-Open Publication No. 59-151020, therefore, proposed is a lead member connected to a heat sensing resistor having a length of a ratio to a diameter of the lead member more than a predetermined ratio. In the Japanese Patent Laid-Open Publication No. 57-201858, it is proposed that the base plate is formed thin and made from a material of low heat conductivity so that the heat transfer is blocked from a portion of the base plate where a film resistor is deposited to a portion where the base plate is fixed.
As described in the above publications, various countermeasures in structure or arrangement of the components of the speed sensing element including resistors are taken in view of the temperature distribution of the thin film resistor in the direction of the intake air, or its temperature distribution based upon the heat transfer from the sensing resistors to the base plate, the lead members and etc.. FIGS. 14 to 16 show a sensing element 100 which is used for an air quantity measuring apparatus proposed in the Japanese Patent Application No. 63-226900 filed by one of the applicants of this application but not disclosed to the public. The sensing element 100 has a base plate 110 formed with a slit 110a, on each side of which there are deposited an intake air temperature sensing resistor 120, a flow speed sensing resistor 130 and a pair of lead members 140, 150 extending from the resistors 120, 130 respectively toward the base end of the base plate 110 as shown in FIG. 14, where a broad arrow indicates the flow direction of the intake air to be measured. According to the sensing element 100, since the flow speed sensing resistor 130 is formed in a letter "U", the size of the flow speed sensing resistor 130, or the width thereof is reduced comparing with the prior resistors formed in series of a letter "S", so that a better temperature distribution of the flow speed sensing resistor 130 is obtained.
However, in the sensing element 100, the temperature distribution on the base plate 110 is made as shown in FIGS. 14 and 15, wherein one-dotted chain lines indicate the temperature of the intake air with the left side in FIG. 14 and the upper side in FIG. 15 set to be higher side in temperature, and solid lines indicate the temperature of a portion of the base plate 110 on which the flow sensing resistor 130 is deposited, and wherein the latter temperature is reduced gradually at the end portions of the base plate 110 in both lateral and longitudinal directions thereof. Therefore, it is necessary to provide some means for compensating the reduction of the temperature at the end portions. Especially, as shown at the right side in FIG. 16, when the flow speed of the intake air is high and the quantity of the intake air is large, the coefficient of the heat transfer to the intake air is large, so that the temperature distribution of the flow speed sensing resistor 130 is made as indicated by "Ho" in FIG. 16. In this FIG. 16, the temperature of the resistor 130 at its downstream side closing to the intake air is reduced rapidly, whereas the temperature at its opposite side is reduced gradually. On the contrary, when the flow speed of the intake air is low and the quantity of the intake air is small, the temperature distribution is made as indicated by "Lo" shown in FIG. 16, where the one-dotted chain line extending vertically indicates the temperature of the intake air with the left side in FIG. 16 set to be higher side in temperature. Since the total quantity of heat transferred from the flow speed sensing resistor 130 is determined by the temperature distribution thereof, a change in the temperature distribution causes an error in sensing the quantity of the intake air. Although this error is small comparing with that in the prior apparatuses, it is desirable to diminish the error further.